Part of “CHAPTER 85 – PHYSIOLOGY OF THE ADRENAL MEDULLA AND THE SYMPATHETIC NERVOUS SYSTEM“

The catecholamines contain a catechol (3,4-dihydroxyphenyl) nucleus (Fig. 85-2). Intracellular events in NE synthesis, turnover, and metabolism are depicted in Figure 85-3. The precursor of the catecholamines—the amino acid tyrosine—is a dietary constituent as well as a product of hepatic metabolism of dietary phenylalanine. Tyrosine is taken up by sympathetic neural tissue and is converted to dihydroxyphenylalanine (DOPA) by the enzyme tyrosine hydroxylase (TH).6,7 The specific enzymatic hydroxylation of tyrosine to DOPA is the rate-limiting step in catecholamine biosynthesis and is end product–inhibited by DOPA, DA, and NE. Although it is known that adrenomedullary chromaffin cells and tissues that possess sympathetic innervation contain TH, it is unknown whether, outside the brain, only sympathetic nerves and adrenomedullary cells contain TH. Current evidence suggests a nonneuronal synthesis of catecholamines.8